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4d47's solution

to Nth Prime in the PHP Track

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Instructions
Test suite
Solution

Given a number n, determine what the nth prime is.

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

Running the tests

  1. Go to the root of your PHP exercise directory, which is <EXERCISM_WORKSPACE>/php. To find the Exercism workspace run

     % exercism debug | grep Workspace
    
  2. Get PHPUnit if you don't have it already.

     % wget --no-check-certificate https://phar.phpunit.de/phpunit.phar
     % chmod +x phpunit.phar
    
  3. Execute the tests:

     % ./phpunit.phar nth-prime/nth-prime_test.php
    

Source

A variation on Problem 7 at Project Euler http://projecteuler.net/problem=7

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

nth-prime_test.php

<?php

require 'nth-prime.php';

class NthPrimeTest extends PHPUnit\Framework\TestCase
{
    public function testFirstPrime()
    {
        $this->assertEquals(2, prime(1));
    }
    public function testSecondPrime()
    {
        $this->assertEquals(3, prime(2));
    }
    public function testSixthPrime()
    {
        $this->assertEquals(13, prime(6));
    }
    public function testBigPrime()
    {
        $this->assertEquals(104743, prime(10001));
    }
    public function testZeroPrime()
    {
        $this->assertEquals(false, prime(0));
    }
}
<?php

function prime(int $n): int {
    if ($n <= 0) {
        return false;
    }

    for ($i = 2, $count = 0; $count < $n; $i++) {
        if (gmp_prob_prime($i)) {
            $count++;
        }
    }
    return $i-1;
}

What can you learn from this solution?

A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that I could read more about to develop my understanding?

Community comments

See what others have said about this solution
about 1 year ago
4d47 says

Okay I know I'm cheating with gmp ;-) This elaborated answer is great read.